Recent light-emitting devices utilizing a semiconductor laser diode (LD) element can provide white light by using a wavelength conversion material such as a fluorescent material that can be excited by, for example, blue light emitted from the LED element to emit yellow light. Specifically, white light can be obtained by mixing the yellow fluorescent light generated from the fluorescent material due to excitation with the blue excitation light originally emitted from the LD element. Favorable white light can be produced by efficiently exciting the fluorescent material with the blue excitation light, and then balancing the blue excitation light and the yellow fluorescent light.
Light-emitting devices for use in a vehicle headlamp should provide light distribution characteristics having both favorable white light characteristics and securement of far distance visibility and wide front irradiation region. The far distance visibility can be achieved by the light distribution property having a large central peak in the light irradiation plane (at the so-called cut-off area). The wider front irradiation region can be achieved by the light distribution property with the light distribution gently lowering from the central region in the light irradiation plane to the periphery.
Japanese Patent Application Laid-Open No. 2004-354495 discloses a light source device including a fluorescent material emitting fluorescent light by excitation light, a concave mirror configured to reflect the light from the fluorescent material, and an LED element or an LD element configured to emit the excitation light.
Japanese Patent Application Laid-Open No. 2010-232044 discloses a vehicle lighting unit including a light-emitting unit that includes an LED light source configured to form a low beam light distribution pattern and a fluorescent material for the LED light source. The fluorescent material configured to emit light upon receipt of the light from the LED light source to (or in the vicinity of) a cut-off area in the light distribution pattern.
Japanese Translation of PCT Patent Application No. 2005-537651 discloses a light-emitting semiconductor, a semispherical transparent lens configured to cover the light-emitting semiconductor, and a fluorescent material layer disposed on or near the surface of the transparent lens, so that the fluorescent material layer is excited by the light from the light-emitting semiconductor to provide fluorescent light.
In the light source device (or light-emitting device) as shown in Japanese Patent Application Laid-Open No. 2004-354495, a large amount of light emitted from the LED element may not be involved in the excitation of the fluorescent material but reflected off the surface of the fluorescent material. Therefore, the amount of reflected excitation light may be more than the amount of the fluorescent light emitted from the fluorescent material, so that it becomes difficult to obtain appropriate white light by adjusting the mixing ratio of the reflected excitation light and the fluorescent light.
In the vehicle lighting unit (or light-emitting device) as shown in Japanese Patent Application Laid-Open No. 2010-232044, the excitation light emitted from the LED light source can excite the fluorescent material, but a large amount of the excitation light cannot excite the fluorescent material and is instead reflected on the surface of the fluorescent material. Therefore, because of the same reason as in the light source device of Japanese Patent Application Laid-Open No. 2004-354495, it becomes difficult to obtain appropriate white light by mixing the reflected excitation light and the fluorescent light.
In the light-emitting semiconductor (or light-emitting device) as shown in Japanese Translation of PCT Patent Application No. 2005-537651, when the excitation light is incident on the fluorescent material layer, the light may be reflected off the rear surface, whereby the light emission efficiency may deteriorate. Further, the fluorescent light component emitted from the fluorescent material may be dispersed in various directions, whereby it becomes difficult to form a light distribution pattern having a large central illuminance peak in the irradiation region and being suitable for use in a vehicle headlamp in particular.
In addition, in the light-emitting device as shown in Japanese Patent Application Laid-Open No. 2004-354495 and Japanese Translation of PCT Patent Application No. 2005-537651, light with a fixed light distribution pattern may only be obtained, and therefore, it is difficult to control the light distribution pattern as desired.